Particular embodiments generally relate to wireless communication and more specifically to phase synchronization in a phased-array or multiple receiver/transmitter systems.
Unless otherwise indicated herein, the approaches described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.
A phased-array transceiver includes multiple antennas that are spatially separated. Due to the distance between antennas, delayed versions of a radio frequency signal are received or transmitted by each antenna. The background will describe a receiver scenario but similar concepts apply to a transmitter. One method of constructing a desired signal from received signals is to adjust the delay of the received signals. However, to add variable delay to each signal received by different antennas requires an analog delay line, where the analog delay line requires a large chip area. Additionally, changing the delay for each received signal is difficult.
Another method of constructing a desired signal is to down convert received signals using accurate phase-shifted versions of a local oscillator (LO) signal. Each received signal may require a different phase of the local oscillator signal. Various conventional techniques for generating different phases of a local oscillator signal are described below.
FIG. 1A depicts a system 100 for generating different phases of a local oscillator signal in accordance with a first conventional technique. System 100 includes a local oscillator (LO) 102, a phase generator 104, and transceiver blocks 106a-106c. Local oscillator 102 is configured to generate a local oscillator signal, such as a high frequency local oscillator signal. Phase generator 104 receives the local oscillator signal, generates all the required phases of the local oscillator signal, and distributes the LO signals with different phases to each transceiver block 106.
Four local oscillator signals with four different phases are generated. Each local oscillator signal is transmitted to each transceiver block 106 through a distribution line. However, having a distribution line for sending each phase to each transceiver block 106 requires a lot of area. Also, each distribution line generally needs to be of an equal length to provide the LO signal to transceiver blocks 106 with accurate phases. For example, a LO signal with a first phase is sent to all three transceiver blocks 106a-106c. If a distribution line to transceiver block 106c is longer than a distribution line to transceiver block 106a, then the LO signal received at transceiver block 106c will have a different phase from the LO signal received at transceiver block 106a. Thus, an accurate phase shifted LO signal has not been provided to each transceiver block 106. An additional problem with using distribution lines is that coupling between the distribution lines may alter the phases of the LO signals.
FIG. 1B depicts a system 150 for generating different phases of a local oscillator signal in accordance with a second conventional technique. System 150 includes a local oscillator (LO) 1-102, phase generators 1-104a-1-104d, and transceiver blocks 1-106a-1-106d. LO 1-102 generates an LO signal. This LO signal is distributed to all transceiver blocks 1-106a-1-106d. Phase generators 1-104a-104b each generate the required phases of the LO signal for transceiver blocks 106a-106d. To accurately generate the different phases precisely, each phase generator 1-104 requires the same phase of the LO signal as a reference. Equal length distribution lines are used to make sure the reference LO signal is received at each phase generator 1-104 with the same phase. For example, system 150 uses a symmetric tree network that distributes the LO signal. As with the distribution lines shown in FIG. 1A, the symmetric tree network shown in FIG. 1B requires a significant amount of chip area.
FIG. 2 depicts a system 200 that uses taps on a transmission line 202 in order to generate different phases of a local oscillator signal. Local oscillator 1-102 generates an LO signal, where the LO signal is passed through transmission line 202. The LO signal is distributed to each transceiver block 1-106a-106d though taps 204a-204d on transmission line 202. Using this structure may save chip area; however, an undesirable phase shift results over the length of transmission line 202.
As shown in FIG. 2, there is a phase shift of ΔΦ in between each tap. Reference LO signals LO1-LO4 will thus have different phases. Having a reference signal with different phases is not desirable because each transceiver block 1-106 needs to have an accurate, known phase of the reference LO signal to process the radio frequency signals. However, the phase shift needs to measured and compensated for or the phase shift needs to be synchronized. However, it is difficult to measure the phase shift or synchronize the phase shift of reference signals when using a transmission line.